The present invention relates, in general, to the arc welding of pipe seams and, in particular, to a new and useful system and method for welding a seam using automatic tracking.
In the metal working field, gas metal arc welding systems are used for welding large diameter pipelines offshore. The equipment is installed on pipe lay barges and utilizes multiple weld heads, i.e. torches, in order to perform the welding of pipe seams. Additionally, as the torches move around the weld groove, each torch oscillates to cover the width of the groove and the seam tracking is controlled during this oscillation as taught by U.S. Pat. No. 4,857,701. At present, the seam tracking is performed by the manual control of each weld head by an operator as disclosed by U.S. Pat. Nos. 3,764,056 and 3,858,026. Automatic seam tracking, such as the tracking disclosed by U.S. Pat. No. 4,525,616, has been developed in order to remove human error from the tracking of weld heads.
Moreover, automatic welding systems have been used for offshore pipelines; and other types of automatic tracking systems are already known.
"Through arc" tracking is one method of automatic tracking that is used in automatic welding. This method senses the change in arc voltage as the arc approaches a sidewall in the weld groove. This information is then electronically processed with respect to the location of the groove. The torch is then guided in the center of the weld groove. Although the method provides for sensing the exact location of the side wall, the system utilized is very complex and requires the addition of sophisticated electronics.
A second method uses a mechanical sensor for mechanical seam trackers which are used in automatic welding. In each case, a mechanical sensor is placed somewhere in the groove, usually ahead of the moving torch, and acts as a guide. The torch, via a mechanical assembly, then follows the sensor. Although the sensor usually provides a simple approach to automatic tracking and does not require as much electronics technology as other known methods, errors are easily introduced due to irregularities in the weld groove resulting from the longitudinal weld seam, weld spatter, grinding and the like.
Another method of automatically tracking a weld seam is through the use of optics. Optical seam trackers typically focus light on the edge of the weld groove or some other known reference and, through opto-electronics, use the location to guide the movement of the weld head down the center of the groove. Although optical technology can be very precise, the optical tracking system is a very complex system requiring the addition of sophisticated electronics and optics.